Ground monitor circuit protection apparatus

ABSTRACT

An apparatus includes a resistor assembly with resistors connected in series between a first terminal and a second terminal of a pilot conductor and/or a ground return of a ground monitor. The pilot conductor injects an electrical signal in a ground of a power cable assembly. A zener diode assembly is connected between a zener connection point and the ground return. The zener connection point is the first or second terminal. The zener diode assembly includes zener diodes connected in series between the zener connection point and the ground return and is sized to clamp a voltage from the zener connection point to the ground return to a value less than a zener threshold voltage. The zener threshold voltage is set above a nominal voltage on the pilot conductor. The components of the apparatus are spaced to prevent an arcing fault current for voltages less than 3000 V.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/740,346 entitled “Ground Monitor Current Sensing” andfiled on Dec. 20, 2012 for Dale Curtis, et al., which is incorporatedherein by reference. U.S. application Ser. No. 13/906,807 entitled“Ground Monitor Current Sensing,” Filed May 31, 2013 for Dale Curtis, etal., and U.S. application Ser. No. ______ entitled “Ground MonitorAccessory Connection,” Filed May 31, 2013 for Dale Curtis, et al. areherein incorporated by reference for all purposes.

FIELD

This invention relates to ground monitors and more particularly relatesto circuit protection within a ground monitor.

BACKGROUND

Mining is a very special environment that is by its very naturehazardous. Mine shafts are very limited physically, often include wetconditions, and can have explosive gases and dust. The mining industryhas a long history of accidents and fatalities. As a result,governmental regulations as well as company policies are geared towardmaking mining safer for those that enter and work in mines. Onegovernmental agency that regulates mining practice in the United Statesis the Mine Safety and Health Administration (“MSHA”). MSHA providesregulations as well as enforcement of the regulations.

Mining equipment is typically large and requires a significant amount ofpower. Mining equipment is also typically portable. To provide power tothe mining equipment, portable power sources are provided in and aroundmines. Due to the high power requirements of mining equipment as well asmines having long shafts, often mining power source have voltages thatare higher than are typically found in industrial situations. It is notuncommon for the mining power sources to provide power with voltagesbeing about 1 kilo volt (“KV”). In addition, due to the portability ofthe mining equipment and power sources, often power is provided usingflexible cables run without conduit. Due to the high voltages, exposedcables, wet conditions, etc., special consideration must be made forsafety of the electrical power systems in mining.

One way to increase safety and reliability of mining power systems is todetermine if grounding conductors are in place and properly connected.When grounding conductors are not properly connected, have shortcircuits, etc., when a fault condition occurs, electrical current canflow through the earth surrounding mine shafts as well as through miningequipment. Current flowing in unintended routes create a shock hazardfor miners both for fault conditions before overcurrent protectionreacts as well as steady state conditions when continuous current flowin unintended paths.

MSHA has provided regulations for monitoring current in groundingconductors. MSHA regulations regarding ground current were changedseveral years ago. Much of the electrical power system equipment in usetoday in mines today does not meet current MSHA requirements found intesting standard 30 of the Code of Federal Regulations (“C.F.R.”)section 75 and 30 C.F.R. section 77 with respect to ground currentmonitoring.

SUMMARY

A circuit protection apparatus for circuit protection in a groundmonitor is disclosed. A system also performs the functions of thecircuit protection apparatus. The circuit protection apparatus includesa resistor assembly connected between a first terminal and a secondterminal. The resistor assembly includes a plurality of resistorsconnected in series between the first terminal and the second terminaland the resistor assembly is connected in a pilot conductor and/or aground return of a ground monitor. The pilot conductor injects anelectrical signal in a grounding conductor of a power cable assembly.The circuit protection apparatus includes a zener diode assemblyconnected between a zener connection point and the ground return of theground monitor. The zener connection point is the first terminal or thesecond terminal. The zener diode assembly includes a plurality of zenerdiodes connected in series between the zener connection point and theground return. The zener diode assembly is sized to clamp a voltage fromthe zener connection point to the ground return to a value of less thanor equal to a zener threshold voltage where the zener threshold voltageis set above a nominal voltage on the pilot conductor. The plurality ofresistors, the plurality of zener diodes, the first terminal and thesecond terminal are spaced to prevent an arcing fault current forvoltages less than or equal to 3000 volts (“V”).

In one embodiment, the resistor assembly is rated to at least 600 watts(“W”). In a further embodiment, the resistor assembly includes twelveresistors each rated to at least 50 W. In another further embodiment,each resistor is rated at 1.5 ohms. In another embodiment, the groundmonitor is within a power source and the power source includes a neutralconnected to ground through a direct connection to the ground or aneutral grounding resistor. The neutral grounding resistor is sized tolimit current for a phase-to-ground fault to less than 25 amperes (“A”).In another embodiment, the resistor assembly is rated to between 10 ohmsand 30 ohms. In another embodiment, the plurality of zener diodes of thezener diode assembly includes ten zener diodes connected in series,where each zener diode is rated with a zener voltage of 4.3 V.

In one embodiment, the resistor assembly includes a quantity ofresistors each physically sized and arranged to prevent arcing acrosseach resistor of the resistor assembly during a transient voltage of3000 V across the resistor assembly. In another embodiment, the zenerdiode assembly includes a quantity of zener diodes each physically sizedand arranged to prevent arcing across each zener diode of the zenerdiode assembly during a transient voltage of 3000 V across the zenerdiode assembly. In another embodiment, the resistor assembly and thezener diode assembly are mounted on a substrate electrically isolatedfrom other components to prevent arcing from one or more of the resistorassembly and the zener diode assembly during a transient voltage of 3000V. In another embodiment, the electrical signal in the pilot conductorand injected in the grounding conductor is a current signal.

Another circuit protection apparatus includes a substrate, a resistorassembly, and a zener diode assembly. The resistor assembly is mountedon the substrate and is connected between a first terminal and a secondterminal. The resistor assembly includes a plurality of resistorsconnected in series between the first terminal and the second terminal.The resistor assembly is connected in a pilot conductor and/or a groundreturn of a ground monitor. The pilot conductor injects a current signalin a grounding conductor of a power cable assembly. The resistorassembly is rated to at least 600 W. Each resistors of the plurality ofresistors is rated at least 600 W divided by the number of resistors inthe plurality of resistors. Each resistor has a rated resistance that isthe same as each of the other resistors in the plurality of resistors.

The zener diode assembly is mounted on the substrate and is connectedbetween a zener connection point and the ground return of the groundmonitor. The zener connection point is the first terminal or the secondterminal. The zener diode assembly includes a plurality of zener diodesconnected in series between the zener connection point and the groundreturn. The zener diode assembly is sized to clamp a voltage from thezener connection point to the ground return to a value of less than orequal to a zener threshold voltage. The zener threshold voltage is setabove a nominal voltage on the pilot conductor. The plurality ofresistors, the plurality of zener diodes, the first terminal and thesecond terminal are spaced from each other and from other components inthe ground monitor to prevent an arcing fault current for voltages lessthan or equal to 3000 V.

In one embodiment, the resistor assembly comprises twelve resistors eachrated to at least 50 W. In another embodiment, each resistor is rated at1.5 ohms. In another embodiment, the resistor assembly is rated tobetween 10 ohms and 30 ohms. In another embodiment, the plurality ofzener diodes of the zener diode assembly includes ten zener diodesconnected in series and each zener diode is rated with a zener voltageof 4.3 V. In another embodiment, the resistor assembly includes aquantity of resistors each physically sized and arranged to preventarcing across each resistor of the resistor assembly during a transientvoltage of 3000 V across the resistor assembly. In another embodiment,the zener diode assembly includes a quantity of zener diodes eachphysically sized and arranged to prevent arcing across each zener diodeof the zener diode assembly during a transient voltage of 3000 V acrossthe zener diode assembly.

A system for circuit protection is disclosed. The system includes aground monitor with a pilot conductor that injects an electric signal ina grounding conductor of a power cable assembly, and a ground returnconnected to the grounding conductor. The system includes a resistorassembly that connects between a first terminal and a second terminal.The resistor assembly includes a plurality of resistors connected inseries between the first terminal and the second terminal. The resistorassembly is connected in the pilot conductor and/or the ground return ofthe ground monitor.

The system includes a zener diode assembly connected between a zenerconnection point and the ground return of the ground monitor. The zenerconnection point is the first terminal or the second terminal. The zenerdiode assembly includes a plurality of zener diodes connected in seriesbetween the zener connection point and the ground return. The zenerdiode assembly is sized to clamp a voltage from the zener connectionpoint to the ground return to a value of less than or equal to a zenerthreshold voltage. The zener threshold voltage is set above a nominalvoltage on the pilot conductor. The plurality of resistors, theplurality of zener diodes, the first terminal and the second terminalspaced to prevent an arcing fault current for voltages less than orequal to 3000 V. In one embodiment, the system includes a power source.The power source provides power to the power cable assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are nottherefore to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating one embodiment of aground monitor system;

FIG. 2 is a schematic block diagram illustrating an apparatus thatincludes an embodiment of the ground monitor apparatus;

FIG. 3A is a schematic block diagram illustrating an apparatus thatincludes an alternate embodiment of the ground monitor apparatus;

FIG. 3B is a schematic block diagram illustrating an apparatus thatincludes another embodiment of the ground monitor apparatus;

FIG. 4 is a schematic block diagram illustrating an apparatus thatincludes a more detailed embodiment of a ground monitor apparatus;

FIG. 5A is a schematic flow chart diagram illustrating one embodiment ofa method for monitoring ground current;

FIG. 5B is a schematic flow chart diagram illustrating anotherembodiment of a method for monitoring ground current;

FIG. 6 is a schematic flow chart diagram illustrating another embodimentof a method for monitoring ground current; and

FIG. 7 is a schematic diagram illustrating one embodiment of a circuitprotection apparatus.

DETAILED DESCRIPTION

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.Where a module or portions of a module are implemented in software, thesoftware portions are stored on one or more computer readable mediums.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing.

More specific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device. Program codeembodied on a computer readable medium may be transmitted using anyappropriate medium, including but not limited to wireless, wireline,optical fiber cable, RF, etc., or any suitable combination of theforegoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

Furthermore, the described features, structures, or characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of programming, software modules, userselections, network transactions, database queries, database structures,hardware modules, hardware circuits, hardware chips, etc., to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention may bepracticed without one or more of the specific details, or with othermethods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Aspects of the present invention are described below with reference toschematic flowchart diagrams and/or schematic block diagrams of methods,apparatuses, systems, and computer program products according toembodiments of the invention. It will be understood that each block ofthe schematic flowchart diagrams and/or schematic block diagrams, andcombinations of blocks in the schematic flowchart diagrams and/orschematic block diagrams, can be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the schematic flowchartdiagrams and/or schematic block diagrams block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the schematic flowchart diagramsand/or schematic block diagrams block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The schematic flowchart diagrams and/or schematic block diagrams in theFigures illustrate the architecture, functionality, and operation ofpossible implementations of apparatuses, systems, methods and computerprogram products according to various embodiments of the presentinvention. In this regard, each block in the schematic flowchartdiagrams and/or schematic block diagrams may represent a module,segment, or portion of code, which comprises one or more executableinstructions for implementing the specified logical function(s).

It should also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in thefigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. Other steps and methods may be conceived that are equivalentin function, logic, or effect to one or more blocks, or portionsthereof, of the illustrated figures.

Although various arrow types and line types may be employed in theflowchart and/or block diagrams, they are understood not to limit thescope of the corresponding embodiments. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the depictedembodiment. For instance, an arrow may indicate a waiting or monitoringperiod of unspecified duration between enumerated steps of the depictedembodiment. It will also be noted that each block of the block diagramsand/or flowchart diagrams, and combinations of blocks in the blockdiagrams and/or flowchart diagrams, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and computerinstructions.

FIG. 1 is a schematic block diagram illustrating one embodiment of aground monitor system 100. The system 100 includes a ground monitorapparatus 102 in a power source 104. The power source 104 includes athree-phase power source 106, a relay 108, and a ground wire device(“GWD”) 126 with a return current sensor 128. The power source 104 iswired to a load 120 with a power cable assembly that includes a firstphase 110, a second phase 112, a third phase 114, and a groundingconductor 118, a neutral grounding resistor 116, a pilot conductor 122,and ground return 130. The load 120 includes a pilot wire diode (“PWD”)124. In another embodiment, the system 100 does not include a PWD 124.The elements of the system 100 are described below.

The ground monitor apparatus 102 monitors current in the groundingconductor 118, and is described in more detail with regard to theapparatus 200 of FIG. 2, the apparatus 300 of FIG. 3A, and the apparatus301 of FIG. 3B. The power source 104, in one embodiment, includes athree-phase power source 106. In one example, the three-phase powersource 106 is a wire-connected source. The three-phase power source 106,in one example, is grounded through a neutral grounding resistor 116. Inanother embodiment, the three-phase power source 106 may be adelta-connected power source. In another embodiment, the power source104 includes another type of power source, such as a single-phase powersource or a direct current (“DC”) power source. The three-phase powersource 106 may include some type of a generator, or may be wired toanother source (not shown). In another embodiment, the power source 104may include multiple power sources. In a particular embodiment, thepower source 104 may be configured for an application in a mine. Thepower source 104, in one embodiment, is configured for miningapplications and in another embodiment is configured to meetrequirements of the Mine Safety and Health Administration (“MSHA”). Oneof skill in the art will recognize other power sources 104 that mayinclude a ground monitor apparatus 102.

In one embodiment, the power source 104 includes a relay 108. In oneexample, the relay 108 is a three-phase contactor. In another example,the relay 108 is a circuit breaker with the remote trip. The relay 108includes a capability to be commanded open by the ground monitorapparatus 102. The relay 108 may include a fault detection module (notshown) that sends a trip signal to the relay 108 on detection of anovercurrent condition. For example, the fault detection module mayinclude current sensing and/or voltage sensing circuits may indicate anovercurrent or fault condition. The fault detection module may have aninverse time characteristic or other common protective relayingcharacteristic and may coordinate with other overcurrent protectiondevices upstream and downstream. The relay 108 may also be opened forother conditions, such as a manual command to open.

In one example, the relay 108 also includes the capability to becommanded open by the ground monitor apparatus 102. For example, theground monitor apparatus 102 may send a command to open the relay 108upon detection of a ground problem. In another embodiment, or the powersource 104 includes a different type of power source, such as a singlephase source, the relay 108 may include a different number of poles. Inanother example, the relay 108 may be a three-pole contactor. One ofskill in the art will recognize other types of relays 108 that may beused in conjunction with the power source 104 that includes a groundmonitor apparatus 102.

In one embodiment, the system 100 includes a power cable assembly wiredto the load 120 with a first phase 110, a second phase 112, a thirdphase 114, and a grounding conductor 118. In other embodiments, thepower cable assembly may include a neutral conductor (groundedconductor) wired to the load and/or the pilot conductor 122. For safety,it is desirable to determine if the grounding conductor 118 is properlyconnected between the power source 104 and the load 120. For example, ifthe grounding conductor 118 is disconnected, has failed, or in some wayis not properly connected between the power source 104 and the load 120,there may be a hidden danger within the system 100. For instance, if ashort circuit occurs and the grounding conductor 118 is not properlyconnected, current may flow through the ground from the load 120 to thepower source 104 and may generate an electric field within the groundthat may cause an electrical hazard for personnel. In one embodiment,the ground monitor apparatus 102 helps to ensure that the groundingconductor 118 is properly connected and functioning.

In one embodiment, the ground monitor apparatus 102 injects a signalinto the grounding conductor 118 to sense an undesirable condition, suchas a high impedance within the grounding conductor 118, a short betweenthe pilot conductor 122 and ground, or other failure in the groundingconductor 118 or in the ground monitor apparatus 102 and associatedcomponents.

In one embodiment, the pilot conductor 122 connects the ground monitorapparatus 102 to the PWD 124 located in the load 120. The PWD 124, inone embodiment, includes a diode, and the pilot conductor 122 is wiredto the anode or cathode of the diode. The opposite terminal (the anodeor cathode) of the diode may be wired to the chassis ground of the load120. The grounding conductor 118 is also connected to the chassis groundof the load 120. In one embodiment, the ground monitor apparatus 102connects an alternating current (“AC”) source to the pilot conductor122. The diode in the PWD 124 rectifies the AC voltage signal so that arectified current signal flows through the pilot conductor 122 andreturns on the grounding conductor 118. If the diode in the PWD 124fails short or if the pilot conductor 122 is shorted to ground, thecurrent signal changes and the ground monitor apparatus 102 is able todetect the change and open the relay 108.

In an alternate embodiment, the load 120 does not include a PWD 124 witha diode and the pilot conductor 122 connects to chassis ground of theload 120 and to the grounding conductor 118. In the embodiment, theground monitor apparatus 102 may use a different current sensing schemethan where the PWD 124 with a diode is included. In another embodiment,a DC voltage source injects current into the pilot conductor 122 and theload does not include a PWD 124 with a diode. In another embodiment, aDC voltage source injects current into the pilot conductor 122 and theload includes a PWD 124 with a diode. In the embodiment, the diode inthe PWD 124 may be a zener diode with the pilot conductor 122 connectedto the cathode the grounding conductor 118 connected to the anode of thezener diode. Various embodiments will be described below with respect tothe apparatuses 200, 300, 301 of FIGS. 2, 3A and 3B.

In one embodiment, the power source 104 includes a GWD 126. The GWD 126,in one embodiment, includes a return current sensor 128. In one example,the return current sensor 128 includes a current transformer that sensescurrent in the grounding conductor 118. In another example, thegrounding conductor 118 is connected to back to back diodes within theGWD 126, and the back-to-back diodes are also connected to the chassisground of the power source 104. The back-to-back diodes, in someembodiments, provide some signal isolation for monitoring injectedsignal in the grounding conductor 118 from the pilot conductor 122. Inaddition, the back-to-back diodes in the GWD 126 help to keep anyvoltage on the grounding conductor 118 to within a diode drop of thechassis ground. In another embodiment, a saturable coil may replace theback-to-back diodes. The saturable coil, in one embodiment, may helpkeep voltage on the grounding conductor 118 within a saturation voltageof the chassis ground. In another embodiment, the GWD 126 does notinclude back-to-back diodes but includes a return current sensor 128 andthe grounding conductor 118 is connected to chassis ground of the powersource 104. In another embodiment, the GWD 126 does not includeback-to-back diodes or a return current sensor 128 and the groundingconductor 118 is connected to chassis ground of the power source 104.One skilled in the art will recognize that various other methods existthat can help limit the voltage potential between the groundingconductor 118 and the chassis ground, while providing some signalisolation for monitoring injected signal in the grounding conductor 118.

FIG. 2 is a schematic block diagram illustrating an apparatus 200 thatincludes an embodiment of the ground monitor apparatus 102. Theapparatus 200 includes a current monitor module 202, a DC detectionmodule 204, a DC threshold module 206, and a trip module 208, which aredescribed below.

The apparatus 200 includes, in one embodiment, a ground monitorapparatus 102 with a current monitor module 202 that monitors current inthe pilot conductor 122. In another embodiment, the current monitormodule 202 monitors current in the ground return 130. The pilotconductor 122 injects current in the grounding conductor 118 of thepower cable assembly connecting the power source 104 to the load 120. AnAC voltage source is connected to the pilot conductor 122 and groundreturn 130. In one example, the voltage source is an AC voltage source.In another example, the voltage source is a DC voltage source.

In one embodiment, the current monitor module 202 includes a currentsensor. For example, the current sensor may be a Hall-effect sensor. Inanother example, the current sensor may be a resistor-type sensor orother component capable of measuring an AC current, a DC component ofthe current, or both in the pilot conductor 122 or ground return 130.The current monitor module 202 may also include other components, suchas op amps, resistors, capacitors, etc. that enable the current monitormodule 202 to produce a signal representative of current through thepilot conductor 122 or ground return 130. One of skill in the art willrecognize other forms of the current monitor module 202 capable ofmeasuring current in the pilot conductor 122 or ground return 130 wherethe current can be DC current, AC current or both.

The apparatus 200 includes, in one embodiment, the ground monitorapparatus 102 with a DC detection module 204 that determines a DCcurrent present in the current monitored by the current monitor module202. In one embodiment where the voltage source is an AC voltage source,the voltage injected by the AC voltage source connected to the pilotconductor 122 and ground return 130 is rectified by the diode in the PWD124. The diode produces a rectified current waveform in the pilotconductor 122 when the system 100 is working properly. The rectifiedcurrent waveform has a DC component that is measurable and significant.The DC detection module 204 determines the DC current present in therectified waveform of the current sensed by the current monitor module202. If a stray DC current is present in the system 100, the stray DCcurrent may add to the DC current that occurs under typical operatingconditions causing the DC current component of the current in the pilotconductor 122 to increase. The DC detection module 204 may detect the DCcurrent component of the typical waveform in addition to any stray DCcurrent that may be present in the pilot conductor 122.

In another embodiment, the loop formed by the pilot conductor 122, thegrounding conductor 118, and the ground return 130 does not include adiode. In the embodiment, the DC detection module 204 determines the DCcurrent present in a sinusoidal waveform present in the pilot conductor122 and ground return 130 and sensed by the current monitor module 202.In the embodiment, under typical operating conditions, current in thepilot conductor 122 or ground return 130 may have a DC current componentthat is low and when a stray DC current is present in the system 100,the stray DC current may add to sinusoidal current. The DC determinationmodule 204 determines the DC current present in the pilot conductor 122or ground return 130 may show an increase in measured DC current due toa stray DC voltage in the system 100.

In another embodiment, the DC detection module 204 detects an average DCcurrent in the pilot conductor 122. For example, the DC detection module204 may detect a DC current present within at least one cycle of theinjected AC voltage waveform. If the AC voltage waveform is 60 hertz(“Hz”), the DC detection module 204 may average the sensed DC currentover one or more cycles of the AC voltage waveform. By sensing DCcurrent over multiple cycles of the AC voltage waveform, the DCdetection module 204 may detect an average DC current that is useful indetermining if a stray DC current has been injected into the system 100and is present in the pilot conductor 122 or ground return 130.

In another embodiment where the voltage source is a DC voltage source,the current monitor module 202 monitors DC current in the pilotconductor 122 or ground return 130 and the DC detection module 204 againdetermines the DC current in the pilot conductor 122 or ground return130. The DC detection module 204, in this embodiment, may merely pass onthe signal from the current monitor module 202 or may modify the signalprior to action by the DC threshold module 206, such as scaling thesignal. The DC detection module 204 may detect DC current present in thepilot conductor 122 or ground return 130 from the DC voltage sourcealong with any stray DC current.

The apparatus 200 includes, in one embodiment, the ground monitorapparatus 102 with a DC threshold module 206 that determines if the DCcurrent in the pilot conductor 122 or ground return 130 is above a DCcurrent threshold. In one embodiment, the DC current thresholdcorresponds to a current that is above a DC current in the pilotconductor 122 that is at a level indicative of an operating conditionwithout a stray DC current component.

For example, where the voltage source is an AC voltage source, the ACvoltage source connected to the pilot conductor 122 may have a root meansquare (“RMS”) voltage of 15 volts. From the AC voltage source throughthe pilot conductor 122, through the PWD 124 (when included), throughthe grounding conductor 118, through the ground return 130 and back tothe AC voltage source, the impedance may be known at least within acertain range such that the current within the pilot conductor 122 maybe a predictable waveform Likewise where the voltage source is a DCvoltage source, a predictable amount of DC current will be present inthe pilot conductor 122 or ground return 130. The DC current of thecurrent within the pilot conductor 122 may then be determined fortypical operating conditions. The DC current threshold may then be setto a value that corresponds to a DC current level in the pilot conductor122 above the DC current level expected under normal operatingconditions. Where the voltage source is an AC voltage source and thesystem 100 does not include the PWD 124 with a diode, a DC current levelmeasured by the current monitor module 202 may be low or zero and the DCcurrent threshold may be set to a different level than where the PWD 124with a diode is included. Thus when the DC detection module 204 detectsa DC current above the DC current threshold, typically a stray DCcurrent is present in the pilot conductor 122.

The apparatus 200 includes, in one embodiment, the ground monitorapparatus 102 with a trip module 208 that opens a contact in response tothe DC threshold module 206 determining that the DC current is above theDC current threshold. The contact disconnects the power source 104 fromthe power cable assembly. In one embodiment, the contact is the relay108 in the power source 104. By opening the relay 108, the trip module208 helps to reduce any hazard caused by the stray DC current present inthe system 100, such as the ground monitor apparatus 102 being renderedinoperable by the stray DC current. In one embodiment, the DC thresholdmodule 206 sends a signal that causes the relay 108 to open. Forexample, where the power source 104 includes a fault detection module(not shown) that opens the relay 108, the trip module 208 may send asignal to the fault detection module and the fault detection module mayopen the relay 108. In another embodiment, the trip module 208encompasses functionality of the fault detection module and trip module208 opens the relay 108 directly. In another embodiment, the trip module208 sends a notification that the DC current is above the DC currentthreshold. For example, the trip module 208 may send a signal to anoperator, to a computer, to a monitoring station, or the like so thatpersonnel using the system 100 may be alerted to the hazard.

In one embodiment, the DC detection module 204 and the DC thresholdmodule 206 include a capability to have multiple DC current thresholdsfor redundancy. For example, the DC detection module 204 and the DCthreshold module 206 may include separate circuits such that if onecircuit fails the other circuit may operate and detect a DC currentabove one of the DC current thresholds.

FIG. 3A is a schematic block diagram illustrating an apparatus 300 thatincludes an alternate embodiment of the ground monitor apparatus 102.The apparatus 300 includes an embodiment of the ground monitor apparatus102 with a current monitor module 202 and a trip module 208 that aresubstantially similar to those described above in relation to theapparatus 200 of FIG. 2. The apparatus 300 also includes a currentanomaly module 302 and an anomaly threshold module 304, which aredescribed below.

In one embodiment, the apparatus 300 includes a current anomaly module302 that determines a current of the current monitored by the currentmonitor module 202 and an anomaly threshold module 304 that determinesif the current determined by the current anomaly module 302 is below acurrent anomaly threshold. The trip module 208 opens a contact inresponse to the anomaly threshold module 304 determining that thecurrent determined by the current anomaly module 302 is below thecurrent anomaly threshold. The current anomaly module 302 may includeone or more amplifiers, resistors, capacitors, and other components thatmay help to provide a signal proportional to current in the pilotconductor 122 or ground return 130 and measured by the current monitormodule 202. The current anomaly module 302 and the anomaly thresholdmodule 304 may be used to determine if a current anomaly exists, such asif the pilot conductor 122 has a short circuit, if the PWD 124 is failedshort, or some other condition that reduces current determined by thecurrent anomaly module 302 to below a current anomaly threshold.

In one embodiment, the current anomaly module 302 determines a peakcurrent of the rectified current typically present in the pilotconductor 122 or ground return 130 when a PWD 124 is included with theload 120. In another embodiment, the current anomaly module 302determines a peak current in the pilot conductor 122 or ground return130 based on an RMS current. This embodiment may be used when the system100 does not include a PWD 124 with a diode, for example. In anotherembodiment where the voltage source is a DC voltage source, the currentanomaly module 302 may determine a DC current in the pilot conductor 122or ground return 130. In the embodiment, the current may be the same asthe DC current determined by the DC detection module 204.

In another embodiment, the current anomaly module 302 may determine anaverage current in the pilot conductor 122 or ground return 130. Inanother embodiment, the current anomaly module 302 may determine that acurrent waveform of the current from the current monitor module 202 isnon-sinusoidal during typical operation and may determine that thecurrent waveform of the current from the current monitor module 202 issinusoidal during atypical operation, such as if the PWD 124 has failedshort. In a typical operational scenario, current from the currentmonitor module 202 may be a rectified sinusoidal waveform, thus having aDC component or average current that is greater than zero. If the PWD124 is shorted or there is a short in the pilot conductor 122, theground return 130 or grounding conductor 118 is disconnected, or someother similar condition, the current anomaly module 302 may determinethat the current from the current monitor module 202 has becomesinusoidal, thus having a DC component or average that is lower thantypical operation and may be below the current anomaly threshold.

The current anomaly threshold may be set to a level that is below acurrent level determined by the current anomaly module 302 under typicaloperating conditions. For example, the current anomaly threshold may bea set to a level that corresponds to a current in the pilot conductor122 or ground return 130 corresponding to impedance within the loopformed by the pilot conductor 122 and the grounding conductor 118 thatis lower than 50 ohms. In one embodiment, 50 ohms corresponds to aparticular requirement of MSHA. As impedance increases within the loopformed by the pilot conductor 122 in the grounding conductor 118,current in the pilot conductor 122 decreases. In one embodiment, thecurrent anomaly threshold is set to level that corresponds to animpedance of 45 ohms. In another embodiment the current anomalythreshold is set to a level that corresponds to an impedance of 48 ohms.

In one embodiment, the current anomaly module 302 and the anomalythreshold module 304 include a capability to have multiple currentanomaly thresholds for redundancy. For example, the current anomalymodule 302 and the anomaly threshold module 304 may include separatecircuits such that if one circuit fails the other circuit may operateand detect a current below one of the current anomaly thresholds. Wherethe voltage source is a DC voltage source, the current anomaly thresholdmay be set to a level below a current in the pilot conductor 122 orground return 130 expected in typical operating conditions.

In another embodiment, the current anomaly module 302 and the anomalythreshold module 304 include a capability to determine that therectified current typically present in the pilot conductor 122 or groundreturn 130 is no longer rectified. In one example, when the diode in thePWD 124 fails short or another fault condition causes current in thepilot conductor 122 or ground return 130 to change such that the currentis not rectified, the current anomaly module 302 reflects a signal thatis indicative of a changing current in the pilot conductor 122 or groundreturn 130. In this condition, current may be less than a current undertypical operating conditions due to the presence of a negative portionof the un-rectified waveform, typically occurring when the diode in thePWD 124 fails or if the pilot conductor 122 is shorted. In an embodimentwhere the voltage source is a DC voltage source, the current anomalymodule 302 may detect a change when the diode in the PWD 124 is shortedor when the pilot conductor 122 is shorted. For example, where the diodein the PWD 124 is a zener diode, the current anomaly module 302 orrelated module may detect a voltage change indicative of the diodefailing short or the pilot conductor 122 being shorted to ground. Theanomaly threshold module 304 or related module may determine that asignal from the current anomaly module 302 or other module detecting avoltage change is such that a diode failure or short circuit conditionexists and may cause the trip module 208 to open the contacts of therelay 108.

FIG. 3B is a schematic block diagram illustrating an apparatus 301 thatincludes another embodiment of the ground monitor apparatus 102. Theapparatus 301 includes, in various embodiments, a current monitor module202, a DC detection module 204, a DC threshold module 206, a trip module208, a current anomaly module 302, and an anomaly threshold module 304,which are substantially similar to those described above in relation tothe apparatuses 200, 300 of FIGS. 2 and 3A. The apparatus 301, invarious embodiments, also includes a return current module 306, and areturn current threshold module 308, which are described below.

The current anomaly module 302 and anomaly threshold module 304, in oneembodiment, may work in conjunction with the DC detection module 204 andthe DC threshold module 206 to monitor multiple problems within thesystem 100 while meeting MSHA requirements, such as MSHA testingstandard 30 of the Code of Federal Regulations (“C.F.R.”) section 75 and30 C.F.R. section 77.

In one embodiment, the apparatus 301 includes a return current module306 that determines a current of the current in the grounding conductor118 and a return current threshold module 308 that determines if thecurrent in the grounding conductor 118 is below a return currentthreshold. The trip module 208 also opens the contact in response to thereturn current threshold module 308 determining that the current in thegrounding conductor 118 is below the return current threshold. In oneembodiment, the return current module 306 receives a current sensesignal from the return current sensor 128. For example, the returncurrent sensor 128 may include a current transformer. In other examples,the return current sensor 128 may include a current sensor of thedifferent type. For example the return current sensor 128 may be aHall-effect sensor. In one embodiment, the return current module 306determines an average current of the current in the grounding conductor118 and the return current threshold module 308 determines if theaverage current in the grounding conductor 118 is below a return currentthreshold.

In one embodiment where the voltage source is an AC voltage source, thereturn current module 306 typically determines that the current in thegrounding conductor 118 is above a particular level that typicallyoccurs when a half wave current signal is present on the groundingconductor 118 when the system 100 includes a PWD 124 or an RMS currentabove a particular level when the system 100 does not include a PWD 124with a diode. In another embodiment where the voltage source is a DCvoltage source, the return current module 306 typically determines thatthe current, which is a DC current, in the grounding conductor 118 isabove a particular DC current level that typically occurs when the DCcurrent injected in the pilot conductor 122 is returned in the groundingconductor 118.

Typically there will be a ground path between the chassis ground of thepower source 104 and the chassis ground of the load 120. For example theground path may include a path through the earth. Typically this groundpath is higher impedance than the grounding conductor 118. However, whenimpedance of the grounding conductor 118 rises, for example when aconnection is loose or when the grounding conductor 118 is broken, asignificant portion of current through the pilot conductor 122 travelsthrough the earth ground path between the two chassis grounds. Thereturn current module 306 and the return current threshold module 308operate in conjunction with the relay 108 to open the contacts of therelay 108 when a significant portion of current in the pilot conductor122 returns through the ground path of the chassis grounds.

In one embodiment, the return current threshold is set to correspond toan impedance of much less than 50 ohms which may correspond to asituation when a significant portion of current in the pilot conductor122 returns through the ground path of the chassis grounds. For example,the return current threshold may be set to be within a range of 50 ohmsto 10 ohms. In one embodiment, the return current threshold is set to16.7 ohms. In another embodiment, the return current threshold is set to20 ohms. In a particular embodiment the return current module 306 andthe return current threshold module 308 include an ability to havemultiple thresholds. For example, the return current module 306 and thereturn current threshold module 308 may include redundant circuits andcomponents with each having a different return current threshold. If onefails, the other may continue to operate. In one embodiment the firstreturn current threshold is set to 16.7 ohms, and a second returncurrent threshold is set to 20 ohms. The impedances of the returncurrent thresholds typically correspond to a decrease in current in thegrounding conductor 118. In one embodiment, the return current thresholdis set to meet a specific test of the MSHA standards mentioned above.The modules 202-208, 302-308 of the apparatus 301 work together toprovide multiple methods of tripping the context of the relay 108 toprotect against a variety of grounding problems and to reduce the riskassociated with various hazards. In addition, the apparatus 301 allowscompliance with current MSHA standards.

FIG. 4 is a schematic block diagram illustrating an apparatus 400 thatincludes a more detailed embodiment of a ground monitor apparatus 102.The apparatus 400 includes an embodiment of the ground monitor apparatus102 in the power source 104, a load 120, a PWD 124, GWD 126 with areturn current sensor 128, a pilot conductor 122, a grounding conductor118, a ground return 130, a current monitor module 202, a DC detectionmodule 204, a DC threshold module 206, the trip module 208, a currentanomaly module 302, an anomaly threshold module 304, a return currentmodule 306, and a return current threshold module 308, which aresubstantially similar to those described in relation to the system 100of FIG. 1 and the apparatuses 200, 300, 301 of FIGS. 2, 3A and 3B. Someelements, such as the power cable assembly and the three-phase powersource 106 are not shown in FIG. 4 for clarity, but may be included in asystem with the apparatus 400 of FIG. 4. The apparatus 400 also includesan AC voltage source 402 substantially similar to the AC voltage sourcedescribed above connected to the pilot conductor 122. The apparatus 400may also include a constant voltage transformer 404, a polarity switch406, a test switch 408, a circuit protection apparatus 410, and anaccessory module 412, which are described below.

In one embodiment, the power source 104 includes an AC voltage source402 connected to feed the pilot conductor 122. In another embodiment,the AC voltage source 402 is located within the ground monitor apparatus102. The AC voltage source 402 typically provides an AC voltage waveformand enough power to send a signal through the pilot conductor 122 andback through the grounding conductor 118 and ground return 130. The ACvoltage source 402, for example, may be a 120 V source. In anotherembodiment, the AC voltage source 402 derives power from the powersource 104. For example, the AC voltage source 402 may be connected toone or more phases (e.g. 110, 112, 114) of the three-phase power source106 and/or to a neutral conductor of the three-phase power source 106.In another embodiment, a DC voltage source (not shown) is connected toinject current in the pilot conductor 122. One of skill in the art willrecognize other ways to provide an AC voltage source 402 capable ofproviding a signal through the pilot conductor 122 or ground return 130.

In one embodiment, the ground monitor apparatus 102 includes a constantvoltage transformer 404. For example, the constant voltage transformer404 may provide a constant 15 V output when the AC voltage source 402 isa voltage that varies. In another embodiment, the ground monitorapparatus 102 includes a polarity switch 406. The polarity switch 406may provide an ability to switch the polarity from the conductors of theoutput of the constant voltage transformer 404. In some alternateembodiments, the polarity switch 406 may switch the polarity of theinput of the constant voltage transformer 404, or conductors of anoutput of the AC voltage source 402. The polarity switch 406 may providea convenient way to switch polarity such that the ground monitorapparatus 102 and associated modules 202-208, 302-308 are able toproperly sense current in the grounding conductor 118. In anotherembodiment, the ground monitor apparatus 102 includes a test switch 408that allows a user to test the ground monitor apparatus 102. Forexample, the test switch 408 may include a resistor such that when abutton is pushed on the test switch 408, the resistor is connected inseries with the pilot conductor 122. In another embodiment, the testswitch 408 is in series with the ground return 130. The resistor may besized appropriately to simulate a condition of a high impedancecondition in the loop formed by the pilot conductor 122 in the groundingconductor 118.

In one embodiment, the apparatus 400 includes a circuit protectionapparatus 410 that helps to lower the risk associated with high voltagetransients and short-circuit currents. The circuit protection apparatus410 will be described in greater detail in relation to the apparatus 410of FIG. 7.

In one embodiment, the apparatus 400 includes an accessory module 412with at least one signal connector that includes a plurality ofconnection points. Each connection point providing access to a signalwithin the ground monitor apparatus 102. In one embodiment, theaccessory module 412 includes a connection point for accessing signalsfor an output of the circuit that monitors current in the pilotconductor 122 or ground return 130, for example, from the currentmonitor module 202. In another embodiment, the accessory module 412includes a connection point for accessing signals for a circuitresponding to a current in the pilot conductor 122 or ground return 130transitioning below an anomaly current threshold, for example from theanomaly threshold module 304. In another embodiment, the accessorymodule 412 includes a connection point for accessing signals for acircuit responding to a current in the grounding conductor 118transitioning below a return current threshold, for example from thereturn current threshold module 308.

In another embodiment, the accessory module 412 includes a connectionpoint for accessing signals for a circuit indicating current status of acircuit to open contacts connecting the power source 104 to the powercable assembly, for example from the trip module 208 and/or relay 108.In another embodiment, the accessory module 412 includes a connectionpoint for accessing one or more signals for a circuit responding to a DCcurrent in the pilot conductor 122 or ground return 130 transitioningabove a DC current threshold. In another embodiment, the accessorymodule 412 includes a connection point for accessing one or more signalsfor a circuit monitoring current in the grounding conductor 118, forexample from the DC threshold module 206.

In one embodiment, the connection points of the accessory module 412 arepart of a first part of a connector assembly configured to connect to asecond part of the connector assembly. The second part of the connectorassembly may be connected to conductors external to the ground monitorapparatus 102. For example, the accessory module 412 may include astandard connector with male or female connection configured to mate toa female or male connector that connects to a wiring harness. Wiringharness may be connected to a computer or other equipment for monitoringvarious points within the ground monitor apparatus 102. In anotherembodiment, the accessory module 412 includes active or passive hardwarethat allows connection to a computer for monitoring various signalswithin the ground monitor apparatus 102. The accessory module 412 mayinclude various circuit components, such as resistors to properly bufferthe connection points from the circuits within the ground monitorapparatus 102.

In one embodiment, the accessory module 412 includes one or moreconnections that may allow input to the ground monitor apparatus 102.For example, the accessory module 412 may include an input to set athreshold, reference signal, trip point, etc. The accessory module 412may facilitate diagnosis of problems within the ground monitor apparatus102 and may include circuits, logic, software, etc. for self-diagnosisor to connect to another device for diagnosis of problems. In anotherembodiment, the inputs may remotely control the ground monitor apparatus102. In another embodiment, the accessory module 412 includes contactsrelated to the trip module 208, such as additional contacts or auxiliarycontacts. For example, the accessory module 412 may include normallyclosed (“NC”) or normally open (“NO”) contacts of a relay that is partof the trip module 208. In another embodiment, the accessory module 412may include auxiliary outputs on a latching trip indicator, which may bea circuit breaker protecting the ground monitor apparatus 102.

In another embodiment, the accessory module 412 includes power supplylines. For example, the accessory module 412 may include a connection tomonitor power supply status or connection points to input power from anexternal power supply. In another embodiment, the power supply lines maybe outputs from a power supply in the ground monitor apparatus 102 forsupplying power to a device external to the ground monitor apparatus102. In another embodiment, the accessory module 412 includes a wirelesscommunication function that receives and transmits wirelessly tocommunicate data to and from various connection points within the groundmonitor apparatus 102. One of skill in the art will recognize other waysto access signals within the ground monitor apparatus 102 and otheruseful signals, connections, contacts, etc. to monitor or connect towithin the ground monitor apparatus 102.

In another embodiment, the accessory module 412 includes power supplylines. For example, the accessory module 412 may include a connection tomonitor power supply status or connection points to input power from anexternal power supply. In another embodiment, the power supply lines maybe outputs from a power supply in the ground monitor apparatus 102 forsupplying power to a device external to the ground monitor apparatus102. In another embodiment, the accessory module 412 includes a wirelesscommunication function that receives and transmits wirelessly tocommunicate data to and from various connection points within the groundmonitor apparatus 102. One of skill in the art will recognize other waysto access signals within the ground monitor apparatus 102 and otheruseful signals, connections, contacts, etc. to monitor or connect towithin the ground monitor apparatus 102.

In one embodiment, the signals of the connection points of the accessorymodule 412 are in addition from signals to the ground monitor apparatus102 (i.e. ground monitor) that interface with the power source 104. Inanother embodiment, the signals of the connection points of theaccessory module 412 include separate signals from the ground monitorapparatus 102 (i.e. ground monitor) that interface with the power source104 in addition to signals indicating status of signals from the groundmonitoring apparatus 102 to the power source 104. In another embodiment,the accessory module 412 is sealed with respect to the ground monitorapparatus 102 such that the seal prevents moisture from entering theground monitor through the accessory module 412 and though an associatedopening in a case of the ground monitor apparatus 102 for the accessorymodule 412. For example, the accessory module 412 may have a waterresistant or waterproof seal to the case of the ground monitor apparatus102. The accessory module 412 may also be water resistant or waterproofto prevent water from entering through the accessory module 412 itself.

FIG. 5A is a schematic flow chart diagram illustrating one embodiment ofa method 500 for monitoring ground current. The method 500 begins andmonitors 502 current in the pilot conductor 122 or ground return 130 ofthe ground monitor apparatus 102. The pilot conductor 122 injectscurrent in the grounding conductor 118 of the power cable assemblyconnecting the power source 104 to the load 120 and a voltage source isconnected to the pilot conductor 122 and ground return 130. In oneembodiment, the current monitor module 202 monitors 502 current in thepilot conductor 122 or ground return 130.

The method 500 determines 504 DC current present in the pilot conductor122 or ground return 130. In one embodiment, the DC detection module 204determines 504 DC current present in the pilot conductor 122 or groundreturn 130. In one example, the DC detection module 204 determines 504DC current present in the pilot conductor 122 or ground return 130. Themethod 500 determines 506 if the DC current is above the DC currentthreshold. If the method 500 determines 506 that the DC current is belowthe DC current threshold, the method 500 returns and monitors 502current in the pilot conductor 122 or ground return 130. If the method500 determines 506 that the DC current is above the DC currentthreshold, the method 500 opens 508 a contact that disconnects the powersource 104 from the power cable assembly, and the method 500 ends. Inone embodiment, the DC threshold module 206 determines 506 if thecurrent in the DC current is above the DC current threshold and the tripmodule 208 opens the contact. The contact may be part of the relay 108.

FIG. 5B is a schematic flow chart diagram illustrating anotherembodiment of a method 501 for monitoring ground current. The method 501begins and monitors 512 current in the pilot conductor 122 or groundreturn 130 of the ground monitor apparatus 102. The pilot conductor 122injects current in the grounding conductor 118 of the power cableassembly connecting the power source 104 to the load 120 and a voltagesource is connected to the pilot conductor 122 and ground return 130. Inone embodiment, the current monitor module 202 monitors 512 current inthe pilot conductor 122 or ground return 130. In one embodiment, thecurrent monitor module 202 monitors 512 current using a Hall-effectsensor.

The method 501 determines 514 current of the monitored current. Forexample, the current anomaly module 302 may determine 514 current of themonitored current. The method 501 determines 516 if the current is belowa current anomaly threshold. If the method 501 determines 516 that thecurrent is above a current anomaly threshold, the method 501 returns andmonitors 512 current in the pilot conductor 122 or ground return 130. Ifthe method 501 determines 516 that the current is below a currentanomaly threshold, the method 501 opens 518 a contact that disconnectsthe power source 104 from the power cable assembly, and the method 501ends. In one embodiment, the current anomaly module 302 determines 514the current and the anomaly threshold module 304 determines 516 if thecurrent is below the current anomaly threshold and the trip module 208opens the contact. The contact may be part of the relay 108.

FIG. 6 is a schematic flow chart diagram illustrating another embodimentof a method 600 for monitoring ground current. The method 600 begins andmonitors 602 current in the pilot conductor 122 or ground return 130 ofthe ground monitor apparatus 102. The method 600 determines 604 DCcurrent present in the pilot conductor 122 or ground return 130 anddetermines 606 if the DC current is above the DC current threshold. Ifthe method 600 determines 606 that the DC current is below the DCcurrent threshold, the method 600 returns and monitors 602 current inthe pilot conductor 122 or ground return 130. If the method 600determines 606 that the DC current is above the DC current threshold,the method 600 opens 608 a contact that disconnects the power source 104from the power cable assembly, and the method 600 ends.

The method 600 also determines 610 current in the pilot conductor 122 orground return 130 and determines 612 if the current is below an anomalycurrent threshold. If method 600 determines 612 that the current isbelow a current anomaly threshold, the method 600 returns and monitors602 current in the pilot conductor 122 or ground return 130. If method600 determines 612 that the current is below a current anomalythreshold, the method 600 opens 608 the contact that disconnects thepower source 104 from the power cable assembly, and the method 600 ends.

The method 600 also monitors 614 current in the grounding conductor 118and determines 616 a current of the current in the grounding conductor118. In one embodiment, the return current module 306 determines 616 thecurrent of the current in the grounding conductor 118. In anotherembodiment, the return current sensor 128 senses return current in thegrounding conductor 118. The method 600 determines 618 if the current inthe grounding conductor 118 is below a return current threshold. If themethod 600 determines 618 that the current in the grounding conductor118 is above a return current threshold, the method 600 returns andmonitors 614 current in the grounding conductor 118. If the method 600determines 618 that the current in the grounding conductor 118 is belowa return current threshold, the method 600 opens 608 the contact thatdisconnects the power source 104 from the power cable assembly, and themethod 600 ends.

FIG. 7 is a schematic diagram illustrating one embodiment of a circuitprotection apparatus 410. The circuit protection apparatus 410 includesa resistor assembly connected between a first terminal 702 and a secondterminal 704. Typically the first and second terminals 702, 704 areconnected in the ground monitor apparatus 102 in series with the pilotconductor 122 and the pilot conductor 122 injects a current signal inthe grounding conductor 118 of the power cable assembly. The resistorassembly includes a plurality of resistors connected in series betweenthe first terminal 702 and the second terminal 704.

In addition, the circuit protection apparatus 410 includes a zener diodeassembly connected between a zener connection point and a terminal 706that is connected to the ground return 130 of the ground monitorapparatus 102. The zener diode assembly includes a plurality of zenerdiodes connected between the zener connection point and the terminal 706that is connected to the ground return 130. The zener connection point,in one embodiment, is the first terminal 702. In another embodiment, thezener connection point is the second terminal 704. The Zener diodeassembly is connected in series between the Zener connection point andthe terminal 706 connected to the ground return 130. The zener diodeassembly is sized to clamp a voltage from the zener connection point tothe ground to a value of less than or equal to a zener threshold. Thezener threshold voltage is typically set above a nominal AC voltage onthe pilot conductor 122.

In one embodiment where the AC voltage on the pilot conductor 122 is 15V RMS with the peak voltage of around 21 V, for example when theconstant voltage transformer 404 is connected to the pilot conductor122, the zener threshold may be set above a peak voltage of the ACvoltage injected on the pilot conductor 122. In another embodiment wherethe voltage source is a DC voltage source, the zener threshold may beset above a DC voltage of the DC voltage source. In one embodiment thezener threshold may be set to 20 V. In alternate embodiment, theresistor assembly is in series with the ground return 130. In theembodiment, the zener diode assembly remains connected with cathodes ofthe zener diodes D1-D10 connected to the pilot conductor 122 and anodesconnected to the ground return 130.

In one embodiment, the plurality of resistors, the plurality of zenerdiodes, the first terminal 702 and the second terminal 704 are spaced toprevent an arcing fault current for voltages less than or equal to anexpected transient voltage. In one embodiment, the expected transientvoltage is a 3000 V. The 3000 V expected transient voltage, in oneembodiment, is derived from an MSHA test. For example, the plurality ofresistors and the plurality of Zener diodes may be arranged to be spacedphysically apart to prevent an arcing fault current for transientvoltages less than the expected transient voltage. In addition theterminals 702, 704, 706, of the circuit protection apparatus 410 mayalso be spaced to prevent an arcing fault current for transient voltagesless than the expected transient voltage.

In another embodiment, the circuit protection apparatus 410 includes aheat sink where the heat sink is isolated from the chassis of the groundmonitor apparatus 102. In another embodiment, the circuit protectionapparatus 410 is isolated from the chassis of the ground monitorapparatus 102 with insulated bushings or other insulation devices. Inanother embodiment, the circuit protection apparatus 410 is isolatedfrom the chassis and is designed to withstand a 3000 transient voltageduring a fault without developing an arcing fault current betweencomponents, between components of the circuit protection apparatus 410and the heat sink, between the circuit protection apparatus 410 and thechassis of the ground monitor apparatus 102, etc.

In one embodiment, a neutral grounding resistor 116 is connected betweenthe neutral connection of the power source 104 and chassis ground of thepower source 104. The neutral grounding resistor 116 is typically sizedbased on available voltages within the power source 104 to limit currentthrough the neutral grounding resistor 116 to about 25 A such that nomore than about 100 V is present in the power source 104 during faultconditions. In one embodiment, the resistor assembly includes 12resistors R1-R12, and each resistor is 1.5 ohms. The total resistance inthe resistor assembly this embodiment is 18 ohms. For a ground monitorapparatus 102 meeting MSHA requirements, a neutral grounding resistor(“NGR”) in the power source 104 may be sized and arranged such thatunder typical fault conditions the expected voltage on the pilotconductor 122 is limited to about 100 V.

Typically the resistance of the circuit protection apparatus 410 is thedominant resistance within the loop formed by the pilot conductor 122,grounding conductor 118, and ground return 130. For 100 volts on thepilot conductor 122, the 18 ohms of the resistor assembly would limitcurrent in the pilot conductor 122 or ground return 130 to around 5.6 A.In one embodiment, the resistance in the resistor assembly is sized toallow overcurrent protection for the pilot conductor 122 to open underfault conditions.

The Zener diode assembly also helps to limit voltage transients andother high-voltage conditions. In one embodiment the Zener diodeassembly includes 10 Zener diodes arranged in series between the Zenerconnection point and the terminal 706 that connects to the ground return130. In the embodiment, the Zener voltage for each Zener diode D1-D10 is4.3 V so that the total Zener voltage for the Zener diode assembly isabout 20 V. For transient voltages, the peak transient voltage may bespread across the Zener diodes D1-D10 of the Zener diode assembly.Having multiple Zener diodes may help to spread the transient voltageacross the Zener diodes so that each diode is not required to be able towithstand the full transient voltage. Where ten Zener diodes D1-D10 areused, typically each Zener diode would see one tenth of the totaltransient voltage. In addition, where multiple resistors are used in theresistor assembly, the total transient voltage may be spread across theresistors R1-R12 of the resistor assembly. In the case where 12resistors R1-R12 are used, typically the voltage across any resistor inthe resistor assembly is about one twelfth of the total transientvoltage.

In one embodiment, the resistor assembly is rated to between about 10ohms and 30 ohms. In another embodiment, each resistor (e.g. R1) is a 50W resistor so that the resistor assembly is rated to 600 watts. In oneembodiment the resistors R1-R12 of the resistor assembly are rated towithstand both transient voltage conditions and fault conditions forexpected transient voltages and fault currents. In one embodiment, theresistor assembly and the zener diode assembly are mounted on asubstrate electrically isolated from other components to prevent arcingfrom the resistor assembly and/or the zener diode assembly during atransient voltage of 3000 V or less. The circuit protection apparatus410 advantageously limits fault current and transient voltages whileallowing overcurrent protection to open during fault conditions. Thecircuit protection apparatus 410 devices in that the circuit protectionapparatus 410 allows the ground monitor apparatus 102 to meet MSHAtests.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A circuit protection apparatus comprising: aresistor assembly connected between a first terminal and a secondterminal, the resistor assembly comprising a plurality of resistorsconnected in series between the first terminal and the second terminal,the resistor assembly connected in one or more of a pilot conductor anda ground return of a ground monitor, the pilot conductor injecting anelectrical signal in a grounding conductor of a power cable assembly;and a zener diode assembly connected between a zener connection pointand the ground return of the ground monitor, the zener connection pointcomprising one of the first terminal and the second terminal, the zenerdiode assembly comprising a plurality of zener diodes connected inseries between the zener connection point and the ground return, thezener diode assembly sized to clamp a voltage from the zener connectionpoint to the ground return to a value of less than or equal to a zenerthreshold voltage, the zener threshold voltage set above a nominalvoltage on the pilot conductor, wherein the plurality of resistors, theplurality of zener diodes, the first terminal and the second terminalspaced to prevent an arcing fault current for voltages less than orequal to 3000 volts (“V”).
 2. The apparatus of claim 1, wherein theresistor assembly is rated to at least 600 watts (“W”).
 3. The apparatusof claim 2, wherein the resistor assembly comprises twelve resistorseach rated to at least 50 W.
 4. The apparatus of claim 3, wherein eachresistor is rated at 1.5 ohms.
 5. The apparatus of claim 1, wherein theground monitor is within a power source, the power source comprising aneutral connected to ground through one of a direct connection to theground and a neutral grounding resistor, the neutral grounding resistorsized to limit current for a phase-to-ground fault to less than 25amperes (“A”).
 6. The apparatus of claim 1, wherein the resistorassembly is rated to between 10 ohms and 30 ohms.
 7. The apparatus ofclaim 1, wherein the plurality of zener diodes of the zener diodeassembly comprises ten zener diodes connected in series, each zenerdiode rated with a zener voltage of 4.3 V.
 8. The apparatus of claim 1,wherein the resistor assembly comprises a quantity of resistors eachphysically sized and arranged to prevent arcing across each resistor ofthe resistor assembly during a transient voltage of 3000 V across theresistor assembly.
 9. The apparatus of claim 1, wherein the zener diodeassembly comprises a quantity of zener diodes each physically sized andarranged to prevent arcing across each zener diode of the zener diodeassembly during a transient voltage of 3000 V across the zener diodeassembly.
 10. The apparatus of claim 1, wherein the resistor assemblyand the zener diode assembly are mounted on a substrate electricallyisolated from other components to prevent arcing from one or more of theresistor assembly and the zener diode assembly during a transientvoltage of 3000 V.
 11. The apparatus of claim 1, wherein the electricalsignal in the pilot conductor and injected in the grounding conductorcomprises a current signal.
 12. A circuit protection apparatuscomprising: a substrate; a resistor assembly mounted on the substrateand connected between a first terminal and a second terminal, theresistor assembly comprising a plurality of resistors connected inseries between the first terminal and the second terminal, the resistorassembly connected in one or more of a pilot conductor and a groundreturn of a ground monitor, the pilot conductor injecting a currentsignal in a grounding conductor of a power cable assembly, the resistorassembly rated to at least 600 watts (“W”), wherein each resistors ofthe plurality of resistors is rated at least 600 W divided by the numberof resistors in the plurality of resistors, wherein each resistor has arated resistance that is the same as each of the other resistors in theplurality of resistors; and a zener diode assembly mounted on thesubstrate and connected between a zener connection point and the groundreturn of the ground monitor, the zener connection point comprising oneof the first terminal and the second terminal, the zener diode assemblycomprising a plurality of zener diodes connected in series between thezener connection point and the ground return, the zener diode assemblysized to clamp a voltage from the zener connection point to the groundreturn to a value of less than or equal to a zener threshold voltage,the zener threshold voltage set above a nominal voltage on the pilotconductor, wherein the plurality of resistors, the plurality of zenerdiodes, the first terminal and the second terminal spaced from eachother and from other components in the ground monitor to prevent anarcing fault current for voltages less than or equal to 3000 volts(“V”).
 13. The apparatus of claim 12, wherein the resistor assemblycomprises twelve resistors each rated to at least 50 W.
 14. Theapparatus of claim 13, wherein each resistor is rated at 1.5 ohms. 15.The apparatus of claim 13, wherein the resistor assembly is rated tobetween 10 ohms and 30 ohms.
 16. The apparatus of claim 12, wherein theplurality of zener diodes of the zener diode assembly comprises tenzener diodes connected in series, each zener diode rated with a zenervoltage of 4.3 V.
 17. The apparatus of claim 12, wherein the resistorassembly comprises a quantity of resistors each physically sized andarranged to prevent arcing across each resistor of the resistor assemblyduring a transient voltage of 3000 V across the resistor assembly. 18.The apparatus of claim 12, wherein the zener diode assembly comprises aquantity of zener diodes each physically sized and arranged to preventarcing across each zener diode of the zener diode assembly during atransient voltage of 3000 V across the zener diode assembly.
 19. Asystem for circuit protection, the system comprising: a ground monitorcomprising a pilot conductor that injects an electric signal in agrounding conductor of a power cable assembly, and a ground returnconnected to the grounding conductor; a resistor assembly connectedbetween a first terminal and a second terminal, the resistor assemblycomprising a plurality of resistors connected in series between thefirst terminal and the second terminal, the resistor assembly connectedin one or more of the pilot conductor and the ground return of theground monitor; and a zener diode assembly connected between a zenerconnection point and the ground return of the ground monitor, the zenerconnection point comprising one of the one of the first terminal and thesecond terminal, the zener diode assembly comprising a plurality ofzener diodes connected in series between the zener connection point andthe ground return, the zener diode assembly sized to clamp a voltagefrom the zener connection point to the ground return to a value of lessthan or equal to a zener threshold voltage, the zener threshold voltageset above a nominal voltage on the pilot conductor, wherein theplurality of resistors, the plurality of zener diodes, the firstterminal and the second terminal spaced to prevent an arcing faultcurrent for voltages less than or equal to 3000 volts (“V”).
 20. Thesystem of claim 19, further comprising a power source, the power sourceproviding power to the power cable assembly.